Today most hydrogen is commercially produced by the method of steam methane reforming, where natural gas stripped of its sulfur content is mixed with steam and passed through heated tubes filled with catalyst to yield a mixture of hydrogen and carbon monoxide. The level of carbon monoxide produced is typically decreased by further converting the carbon monoxide to carbon dioxide through reaction of the carbon monoxide with steam in a water gas shift reactor to yield a hydrogen/carbon dioxide mixture. Pure hydrogen is separated from the resulting hydrogen/carbon dioxide mixture in a pressure swing adsorption unit.
There are a variety of drawbacks associated with using the above method, including, but not limited to, the production of steam produced by waste heat boilers used to cool the hydrogen/carbon monoxide mixture from the steam methane reformer reactor to the water gas shift reactor (from approximately 850° C. to approximately 350° C.), the emission of low-pressure carbon dioxide mixed with nitrogen and water vapor, and cost of three process units required for hydrogen production, namely, the steam methane reformer reactor, water gas shift reactor, and pressure swing adsorption unit.
Using a membrane reformer instead of a steam methane reformer reactor and water gas shift reactor reduces the amount of flue gas and carbon dioxide generated in the hydrogen production process and makes heat integration easier. In addition, a membrane reformer produces less—or zero—steam since it does not require cooling syngas from the steam methane reformer reactor (approximately 850° C.) to the water gas shift reactor (approximately 350° C.). This cooling is usually accomplished by passing the hot syngas through a waste heat boiler. In a membrane reformer, both the reforming and shift reactions may be conducted in the same reactor at a temperature of from about 500° C. to about 600° C.
However, with current palladium or palladium alloy-based membrane reformers, the hydrogen product pressure is too low for practical use. The product pressure depends upon the hydrogen partial pressure on the process side, the membrane permeance and surface area, as well as the required hydrogen flow. Current palladium based membranes are deposited on porous tubes, such as alumina or stainless steel, with a typical outer diameter of 5 mm or more. See, e.g. U.S. Pat. No. 7,175,694.
The use of hollow fiber membranes for separation of mixtures of liquids and gases is well developed and commercially very important art. Such membranes are traditionally composed of a polymeric composition through which the components from the mixture to be separated are able to travel at different rates under a given set of driving force conditions, e.g. trans-membrane pressure and concentration gradients. Examples are the desalination of water by reverse osmosis, separation of water/ethanol mixtures by pervaporation, separation of hydrogen from refinery and petrochemical process streams, enrichment of oxygen or nitrogen from air, and removal of carbon dioxide from natural gas streams. In each separation, the membranes must withstand the conditions of the application, and must provide adequate flux and selectivity in order to be economically attractive.
The use of hollow fibers is recognized to have advantages over flat film or planar membranes due to the large membrane surface area for separation within a specific volume of apparatus. The success of polymeric hollow fiber membranes has in part been due to the ability to produce fibers of extremely small diameter—in some cases, the diameter of a human hair (about 80 microns). The ability to utilize small diameter fibers allows for extremely high module surface areas per system volume, which allows for the processing of high volumes of fluid in a smaller system size.
In certain applications where high chemical resistance and operation at high temperature and pressure are desired, such as in a membrane reformer, polymeric membranes have not been suitable for use because of the degradation of membrane performance during operation. Inorganic or ceramic membranes have been successfully made in flat or planar shapes and large cylindrical tubes (>1 cm diameter), but have had limited commercial success because of their relatively low surface area compared to small diameter hollow fiber membranes. Production of small diameter ceramic hollow fibers has been problematic with respect to strength of the precursor fiber (sometimes referred to as a “green” fiber) and the final fiber after sintering.
Such hollow fibers are typically made from a suspension of inorganic particles in a liquid medium with a suitable binder to form a paste, which is subsequently extruded through an annular die to form a precursor hollow fiber. After removal of the liquid dispersion medium, the precursor fiber is sintered at elevated temperature to consolidate the individual particulate structure into a micro-porous structure.
For the production of small diameter inorganic fibers, it has been found to be beneficial to incorporate a polymeric binder in the paste to strengthen the nascent fiber. The polymer is typically soluble in the liquid medium of the paste. After the paste is extruded to form a nascent hollow fiber, the polymer solution in the interstices between the inorganic particles is coagulated to solidify the polymer by passing the nascent fiber into a liquid bath containing a coagulating fluid. Alternatively, the liquid can be removed by evaporation to solidify the polymer. The resulting polymeric/inorganic precursor fiber has considerably greater strength and ductility than exhibited in the absence of a polymeric binder.